{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# ResNet\n",
    "当大家还在惊叹 GoogLeNet 的 inception 结构的时候，微软亚洲研究院的研究员已经在设计更深但结构更加简单的网络 ResNet，并且凭借这个网络子在 2015 年 ImageNet 比赛上大获全胜。\n",
    "\n",
    "ResNet 有效地解决了深度神经网络难以训练的问题，可以训练高达 1000 层的卷积网络。网络之所以难以训练，是因为存在着梯度消失的问题，离 loss 函数越远的层，在反向传播的时候，梯度越小，就越难以更新，随着层数的增加，这个现象越严重。之前有两种常见的方案来解决这个问题：\n",
    "\n",
    "1.按层训练，先训练比较浅的层，然后在不断增加层数，但是这种方法效果不是特别好，而且比较麻烦\n",
    "\n",
    "2.使用更宽的层，或者增加输出通道，而不加深网络的层数，这种结构往往得到的效果又不好\n",
    "\n",
    "ResNet 通过引入了跨层链接解决了梯度回传消失的问题。\n",
    "\n",
    "![](https://ws1.sinaimg.cn/large/006tNc79ly1fmptq2snv9j30j808t74a.jpg)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这就普通的网络连接跟跨层残差连接的对比图，使用普通的连接，上层的梯度必须要一层一层传回来，而是用残差连接，相当于中间有了一条更短的路，梯度能够从这条更短的路传回来，避免了梯度过小的情况。\n",
    "\n",
    "假设某层的输入是 x，期望输出是 H(x)， 如果我们直接把输入 x 传到输出作为初始结果，这就是一个更浅层的网络，更容易训练，而这个网络没有学会的部分，我们可以使用更深的网络 F(x) 去训练它，使得训练更加容易，最后希望拟合的结果就是 F(x) = H(x) - x，这就是一个残差的结构\n",
    "\n",
    "残差网络的结构就是上面这种残差块的堆叠，下面让我们来实现一个 residual block"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T12:56:06.772059Z",
     "start_time": "2017-12-22T12:56:06.766027Z"
    },
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import sys\n",
    "sys.path.append('..')\n",
    "\n",
    "import numpy as np\n",
    "import torch\n",
    "from torch import nn\n",
    "import torch.nn.functional as F\n",
    "from torch.autograd import Variable\n",
    "from torchvision.datasets import CIFAR10"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T12:47:49.222432Z",
     "start_time": "2017-12-22T12:47:49.217940Z"
    },
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def conv3x3(in_channel, out_channel, stride=1):\n",
    "    return nn.Conv2d(in_channel, out_channel, 3, stride=stride, padding=1, bias=False)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 27,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:14:02.429145Z",
     "start_time": "2017-12-22T13:14:02.383322Z"
    },
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class residual_block(nn.Module):\n",
    "    def __init__(self, in_channel, out_channel, same_shape=True):\n",
    "        super(residual_block, self).__init__()\n",
    "        self.same_shape = same_shape\n",
    "        stride=1 if self.same_shape else 2\n",
    "        \n",
    "        self.conv1 = conv3x3(in_channel, out_channel, stride=stride)\n",
    "        self.bn1 = nn.BatchNorm2d(out_channel)\n",
    "        \n",
    "        self.conv2 = conv3x3(out_channel, out_channel)\n",
    "        self.bn2 = nn.BatchNorm2d(out_channel)\n",
    "        if not self.same_shape:\n",
    "            self.conv3 = nn.Conv2d(in_channel, out_channel, 1, stride=stride)\n",
    "        \n",
    "    def forward(self, x):\n",
    "        out = self.conv1(x)\n",
    "        out = F.relu(self.bn1(out), True)\n",
    "        out = self.conv2(out)\n",
    "        out = F.relu(self.bn2(out), True)\n",
    "        \n",
    "        if not self.same_shape:\n",
    "            x = self.conv3(x)\n",
    "        return F.relu(x+out, True)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "我们测试一下一个 residual block 的输入和输出"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 28,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:14:05.793185Z",
     "start_time": "2017-12-22T13:14:05.763382Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "input: torch.Size([1, 32, 96, 96])\n",
      "output: torch.Size([1, 32, 96, 96])\n"
     ]
    }
   ],
   "source": [
    "# 输入输出形状相同\n",
    "test_net = residual_block(32, 32)\n",
    "test_x = Variable(torch.zeros(1, 32, 96, 96))\n",
    "print('input: {}'.format(test_x.shape))\n",
    "test_y = test_net(test_x)\n",
    "print('output: {}'.format(test_y.shape))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 29,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:14:11.929120Z",
     "start_time": "2017-12-22T13:14:11.914604Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "input: torch.Size([1, 3, 96, 96])\n",
      "output: torch.Size([1, 32, 48, 48])\n"
     ]
    }
   ],
   "source": [
    "# 输入输出形状不同\n",
    "test_net = residual_block(3, 32, False)\n",
    "test_x = Variable(torch.zeros(1, 3, 96, 96))\n",
    "print('input: {}'.format(test_x.shape))\n",
    "test_y = test_net(test_x)\n",
    "print('output: {}'.format(test_y.shape))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "下面我们尝试实现一个 ResNet，它就是 residual block 模块的堆叠"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 30,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:27:46.099404Z",
     "start_time": "2017-12-22T13:27:45.986235Z"
    },
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class resnet(nn.Module):\n",
    "    def __init__(self, in_channel, num_classes, verbose=False):\n",
    "        super(resnet, self).__init__()\n",
    "        self.verbose = verbose\n",
    "        \n",
    "        self.block1 = nn.Conv2d(in_channel, 64, 7, 2)\n",
    "        \n",
    "        self.block2 = nn.Sequential(\n",
    "            nn.MaxPool2d(3, 2),\n",
    "            residual_block(64, 64),\n",
    "            residual_block(64, 64)\n",
    "        )\n",
    "        \n",
    "        self.block3 = nn.Sequential(\n",
    "            residual_block(64, 128, False),\n",
    "            residual_block(128, 128)\n",
    "        )\n",
    "        \n",
    "        self.block4 = nn.Sequential(\n",
    "            residual_block(128, 256, False),\n",
    "            residual_block(256, 256)\n",
    "        )\n",
    "        \n",
    "        self.block5 = nn.Sequential(\n",
    "            residual_block(256, 512, False),\n",
    "            residual_block(512, 512),\n",
    "            nn.AvgPool2d(3)\n",
    "        )\n",
    "        \n",
    "        self.classifier = nn.Linear(512, num_classes)\n",
    "        \n",
    "    def forward(self, x):\n",
    "        x = self.block1(x)\n",
    "        if self.verbose:\n",
    "            print('block 1 output: {}'.format(x.shape))\n",
    "        x = self.block2(x)\n",
    "        if self.verbose:\n",
    "            print('block 2 output: {}'.format(x.shape))\n",
    "        x = self.block3(x)\n",
    "        if self.verbose:\n",
    "            print('block 3 output: {}'.format(x.shape))\n",
    "        x = self.block4(x)\n",
    "        if self.verbose:\n",
    "            print('block 4 output: {}'.format(x.shape))\n",
    "        x = self.block5(x)\n",
    "        if self.verbose:\n",
    "            print('block 5 output: {}'.format(x.shape))\n",
    "        x = x.view(x.shape[0], -1)\n",
    "        x = self.classifier(x)\n",
    "        return x"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "输出一下每个 block 之后的大小"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:28:00.597030Z",
     "start_time": "2017-12-22T13:28:00.417746Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "block 1 output: torch.Size([1, 64, 45, 45])\n",
      "block 2 output: torch.Size([1, 64, 22, 22])\n",
      "block 3 output: torch.Size([1, 128, 11, 11])\n",
      "block 4 output: torch.Size([1, 256, 6, 6])\n",
      "block 5 output: torch.Size([1, 512, 1, 1])\n",
      "output: torch.Size([1, 10])\n"
     ]
    }
   ],
   "source": [
    "test_net = resnet(3, 10, True)\n",
    "test_x = Variable(torch.zeros(1, 3, 96, 96))\n",
    "test_y = test_net(test_x)\n",
    "print('output: {}'.format(test_y.shape))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 32,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:29:01.484172Z",
     "start_time": "2017-12-22T13:29:00.095952Z"
    },
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "from utils import train\n",
    "\n",
    "def data_tf(x):\n",
    "    x = x.resize((96, 96), 2) # 将图片放大到 96 x 96\n",
    "    x = np.array(x, dtype='float32') / 255\n",
    "    x = (x - 0.5) / 0.5 # 标准化，这个技巧之后会讲到\n",
    "    x = x.transpose((2, 0, 1)) # 将 channel 放到第一维，只是 pytorch 要求的输入方式\n",
    "    x = torch.from_numpy(x)\n",
    "    return x\n",
    "     \n",
    "train_set = CIFAR10('./data', train=True, transform=data_tf)\n",
    "train_data = torch.utils.data.DataLoader(train_set, batch_size=64, shuffle=True)\n",
    "test_set = CIFAR10('./data', train=False, transform=data_tf)\n",
    "test_data = torch.utils.data.DataLoader(test_set, batch_size=128, shuffle=False)\n",
    "\n",
    "net = resnet(3, 10)\n",
    "optimizer = torch.optim.SGD(net.parameters(), lr=0.01)\n",
    "criterion = nn.CrossEntropyLoss()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 33,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2017-12-22T13:45:00.783186Z",
     "start_time": "2017-12-22T13:29:09.214453Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Epoch 0. Train Loss: 1.437317, Train Acc: 0.476662, Valid Loss: 1.928288, Valid Acc: 0.384691, Time 00:00:44\n",
      "Epoch 1. Train Loss: 0.992832, Train Acc: 0.648198, Valid Loss: 1.009847, Valid Acc: 0.642405, Time 00:00:48\n",
      "Epoch 2. Train Loss: 0.767309, Train Acc: 0.732617, Valid Loss: 1.827319, Valid Acc: 0.430380, Time 00:00:47\n",
      "Epoch 3. Train Loss: 0.606737, Train Acc: 0.788043, Valid Loss: 1.304808, Valid Acc: 0.585245, Time 00:00:46\n",
      "Epoch 4. Train Loss: 0.484436, Train Acc: 0.834499, Valid Loss: 1.335749, Valid Acc: 0.617089, Time 00:00:47\n",
      "Epoch 5. Train Loss: 0.374320, Train Acc: 0.872922, Valid Loss: 0.878519, Valid Acc: 0.724288, Time 00:00:47\n",
      "Epoch 6. Train Loss: 0.280981, Train Acc: 0.904212, Valid Loss: 0.931616, Valid Acc: 0.716871, Time 00:00:48\n",
      "Epoch 7. Train Loss: 0.210800, Train Acc: 0.929747, Valid Loss: 1.448870, Valid Acc: 0.638548, Time 00:00:48\n",
      "Epoch 8. Train Loss: 0.147873, Train Acc: 0.951427, Valid Loss: 1.356992, Valid Acc: 0.657536, Time 00:00:47\n",
      "Epoch 9. Train Loss: 0.112824, Train Acc: 0.963895, Valid Loss: 1.630560, Valid Acc: 0.627769, Time 00:00:47\n",
      "Epoch 10. Train Loss: 0.082685, Train Acc: 0.973905, Valid Loss: 0.982882, Valid Acc: 0.744264, Time 00:00:44\n",
      "Epoch 11. Train Loss: 0.065325, Train Acc: 0.979680, Valid Loss: 0.911631, Valid Acc: 0.767009, Time 00:00:47\n",
      "Epoch 12. Train Loss: 0.041401, Train Acc: 0.987952, Valid Loss: 1.167992, Valid Acc: 0.729826, Time 00:00:48\n",
      "Epoch 13. Train Loss: 0.037516, Train Acc: 0.989011, Valid Loss: 1.081807, Valid Acc: 0.746737, Time 00:00:47\n",
      "Epoch 14. Train Loss: 0.030674, Train Acc: 0.991468, Valid Loss: 0.935292, Valid Acc: 0.774031, Time 00:00:45\n",
      "Epoch 15. Train Loss: 0.021743, Train Acc: 0.994565, Valid Loss: 0.879348, Valid Acc: 0.790150, Time 00:00:47\n",
      "Epoch 16. Train Loss: 0.014642, Train Acc: 0.996463, Valid Loss: 1.328587, Valid Acc: 0.724387, Time 00:00:47\n",
      "Epoch 17. Train Loss: 0.011072, Train Acc: 0.997363, Valid Loss: 0.909065, Valid Acc: 0.792919, Time 00:00:47\n",
      "Epoch 18. Train Loss: 0.006870, Train Acc: 0.998561, Valid Loss: 0.923746, Valid Acc: 0.794403, Time 00:00:46\n",
      "Epoch 19. Train Loss: 0.004240, Train Acc: 0.999500, Valid Loss: 0.877908, Valid Acc: 0.802314, Time 00:00:46\n"
     ]
    }
   ],
   "source": [
    "train(net, train_data, test_data, 20, optimizer, criterion)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "ResNet 使用跨层通道使得训练非常深的卷积神经网络成为可能。同样它使用很简单的卷积层配置，使得其拓展更加简单。\n",
    "\n",
    "**小练习：  \n",
    "1.尝试一下论文中提出的 bottleneck 的结构   \n",
    "2.尝试改变 conv -> bn -> relu 的顺序为 bn -> relu -> conv，看看精度会不会提高**"
   ]
  }
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